How High Will Sea Levels Go by 2100?

Below:

Next story in Science

Rising sea level is among the most potentially catastrophic
effects of human-caused climate change. Increases in sea level
strengthen the destructive power of storms, and threaten to swamp
major coastal cities, as well as small-island and low-lying
nations. In the United States alone, more than 8 million people
live in areas at risk of coastal flooding.

During the 20th century, global sea levels crept up by about 6.7
inches (17 centimeters), and the rate of increase appears to have
accelerated since the 19th century.

This century, scientists expect to see sea levels continuing to
rise as more warming causes water to expand and melts ice that
flows into the ocean. But just
how high sea levels will go in the next 87 years is a wide
open question. A soon-to-be released report won't have definitive
answers, but it is
expected to offer a range of projections drawn from current
research.

On Friday, Sept. 27, the
Intergovernmental Panel on Climate Change (IPCC) is scheduled
to release a summary of the physical sciences section of its
Assessment Report 5. The numbers in the IPCC’s assessment reports
serve as a reference point for decision makers and for the
general understanding of climate change.

Six years ago, an earlier version of the IPCC's
report estimated average sea levels could rise by somewhere
between 7 inches (18 centimeters) and 1.9 feet (0.59 meters) by
2100. But scientists lacked key information when they put this
report together, and work done since indicates that
end-of-the-century sea levels could be higher.

"The key point about those projections is they included only part
of the story,” Chris Little, an associate research scholar at
Princeton University, said of the sea level projections in 2007.
Namely, the authors could not fully consider potential
contributions from
ice sheets covering Greenland and Antarctica.

Scientists know that these ice sheets could give a large boost to
sea levels because of the sheer amount of water locked up in
them. These massive layers of ice take up water from the oceans
through snowfall (water evaporates from the oceans and turns to
precipitation in the atmosphere) and return it through melt, but
scientists didn’t understand aspects of this process well enough
to fully account for how climate change might affect it.

"It is truly an ongoing process," Little said. "The information
is going to continue to get better over time."

The
IPCC projections have relied on scientists' understanding of
physical processes that contribute to sea level change. Another
type of model has yielded higher future sea level projections,
topping out at 2 m (6.6 feet). These semi-empirical models rely
heavily on the historical relationship between changes in
temperatureand sea level to look into the future.

"The big argument in the scientific community is, 'How much is
this going to look like past events?" because this really is
different," said Josh Willis, a climate scientist at NASA’s Jet
Propulsion Laboratory in Pasadena, Calif. "We are dumping carbon
dioxide into the atmosphere faster than anything done in millions
of years, and the temperature is changing really rapidly."

What’s more, after the peak of the last ice age, much more ice
was available to melt and raise sea levels than at present, he
said.

But the other alternative, physics-based models, still don't
account for all of the processes that can influence changing sea
levels, potentially underestimating the impact. "So we are stuck
between a rock and a hard place," Willis said.

"The 2 meters by 2100 is cited a lot, but if you ask scientists
what they think of that number, they say it is probably a little
high, maybe 1.5 meters [4.9 feet] is more like an upper bound,"
Willis said.